A technique to provide an in-vehicle network with more robust message authentication against a replay attack has been proposed (see Patent Literature 1). Robustness is increased by counting the number of times a message is transmitted for each message identifier (hereinafter, referred to as CANID) in a CAN protocol, and by using a count value to generate a message authentication code (hereinafter, abbreviated to a MAC) in each ECU. CAN is registered trademark.